All the above could mean that this wasn't a traditional wedding from start, possibly taking after polyamorous weddings but without making too obvious, both because out- and In-Universe reasons. The former because it keeps the audience on their toes, the latter because Japan is still a very traditionalist country and it wouldn't look good for the daughters of a prominent doctor to make their polygamous marriage public.
This would explain why Mr Nakano has to be dragged by Futarou's father to the wedding, and why someone as meticulous as the groom would forget the wedding ring at home. After all, it's a lot more of easier to lose track of one ring in a set of five (six if we count his) than just the one.
- Potentially jossed by Word of God. In a interview, Negi Haruba seemed to have iterated that there's only one bride and expressed disappointment that many fans seemed to want a harem ending.
- Outright jossed; those extra wedding dresses were only there for one last Quintuplet Game.
Maeda is present, so we know that they stayed friends. Takeda seems closer to Fuutarou, and even inspired Fuutarou to change his goal regarding the Quints, therefore it would make sense for Fuutarou to choose him as the best man.
- The identity of the bride is left completely ambiguous and no hints are actually given as to who it might be, allowing fans of any quintuplet to imagine her being the bride.
- The series has Multiple Endings, each with a different quintuplet as the bride, with each ending being equally plausible, so there is no one "canon" or Golden Ending. Fans can then choose which ending they want best, based on which quintuplet they want the bride to be.
- Jossed.
Since all five Nakano sisters looked the same when they were younger, and Fuutarou was unaware that there was more than one of them, he would have been oblivious to the sisters’ differing personalities and chalked up any changes in “Rena” that occurred each time the girls switched out to different moods or (in hindsight) even his own faulty memory. Plus, by tying in each girl to Fuutarou’s past, it once again makes the “playing field” wide open and essentially makes the “mystery” element one big red herring, while not being any less thematically valid: “Rena” (or rather, a “Rena”) really is the girl he will marry; it just doesn’t narrow things down much, or even at all.
- Partly confirmed. The sister that Fuutarou first met was Yotsuba, Ichika also interacted with him in a hotel room (by pretending to be the same girl), and Itsuki acted as the high school Rena at Yotsuba's request. There's no indication that Nino or Miku were involved with him in the past.
- Other people also have difficulties telling them apart. It might well be that the differences in color are exaggerated so that the we, the viewers, can better tell them apart.